Where is the design of the tallest building in the world?

This question is a bit confusing grammatically. Did you mean where is the tallest building? If so, the tallest building, as of August 2011, is the Burj Khalifa in Dubai (Dubai is a city in the country called the United Arab Emirates).

If you meant who designed this building, the answer is that it was designed by the architecture firm called "Skidmore, Owings, and Merrill" - which is an American architecture firm with offices around the world.